1. Field of the Invention
The present invention is directed generally to non-aqueous electrochemical cells and, more particularly, to improvements with respect to over-pressure problems occurring during charging of secondary or rechargeable type cells and during high rate discharge in primary or secondary cells.
2. Description of the Related Art
Non-aqueous active metal cells have been utilized to achieve much higher energy densities or energy-to-weight ratios in both primary and secondary (rechargeable) applications. Potential uses and desirability of cells having such high energy density has created a great deal of interest in using the technology in many areas including those where safety is of prime concern. One traditional drawback long associated with and still plaguing lithium or other high energy cells has been concern with safety surrounding the discharge of and, in the case of secondary cells, the recharging of such cells because of their history of runaway reaction and over-pressurization problems.
Cells of the class typically consist of a light, strongly reducing metallic anode, normally lithium or an alloy of lithium, an electrolyte system including an aprotic, non-aqueous solvent depolarizer into which has been dissolved an appropriate quantity of a salt of the anode metal to form a conductive solution, and an oxidizing agent as the cathode active material. In some cases, the electrolyte solvent or co-solvent also acts as the active cathode material which is subsequently reduced at a porous carbon electrode.
Primary lithium cells (both active and reserve) have been in use for quite some time; however, rechargeable lithium cells have been made practical only relatively recently with improvements in lithium cycling efficiency. The electrolyte systems in secondary lithium cells have been predominantly ether- or ester-based organic electrolyte solutions. The ether-based solutions include, for example, the electrolyte salt LiAsF.sub.6 dissolved in 2-methyl tetrahydrofuran (2-methyl THF). Ester-based solutions generally have higher conductivities and so appear to be more promising overall. Typical esters include methyl formate (HCOOCH.sub.3) and methyl acetate (CH.sub.3 COOCH.sub.3). The electrolyte salt normally combined with methyl formate or methyl acetate is LiAsF.sub.6 which may contain an amount of LiBF.sub.4. The cathode material is normally a transition metal oxide such as V.sub.2 O.sub.5, or the like.
The typical active metal cell of the class of interest is hermetically sealed, and many primary and almost all rechargeable cells using organic solvent electrolytes incorporate a venting system designed to prevent case rupture of the cell or battery should abnormally high internal pressure occur. The abnormally high pressure may result from abuse of the cell such as from overheating, external shorting, causing a very high discharge rate, overcharging and other intentional or unintentional practices. The over-pressure condition may also be caused by internal failure such as shorts due to dendrite formation, for example. Present venting systems are of the external pressure relief type and function quite well to prevent explosions or cell ruptures in extreme circumstances. Operation of the vent mechanisms, however, results in the release of hot, possibly flammable or toxic gases, or the like, which is also undesirable. Cell over-pressure, which occurs typically as part of an end-of-life failure mode in rechargeable lithium cells, involves dendritic shorts (which eventually form on charging even at low currents) causing degradation of the electrolyte. Pressurization and eventual venting occurs often without any indication that a problem exists as it is usually not otherwise predicted.
Several schemes have been devised which provide mechanical devices for opening cell circuits based on pressure and/or heat. One such spring-loaded or automatic reset system is illustrated and described in Jost, et al. U.S. Pat. No. 3,373,057. There, a spring-biased, snap-acting mechanism within the cell housing operates to open the charging circuit during an over-pressure condition. The contact is designed to reclose once the pressure is reduced so that the biasing spring is able to restore the original configuration and circuit continuity. Other systems are shown in Yanagisawa U.S. Pat. No. 4,191,870 and Johnson, et al. U.S. Pat. No. 4,573,398. In addition, nonresettable or one-time operating mechanisms are known such as that shown in Tucholski U.S. Pat. No. 4,028,478, in which a bevelled spring washer operates to break a connection upon expansion of the cell container.
Whereas, there are known prior devices which operate to separate cell circuitry in response to over-pressure conditions which do not require external venting of the cells, there yet remains the need for a simplified positive system which both visibly displays the status of the circuitry and provides a positive disconnect within the cell.